Not Applicable.
The present embodiments relate to high-speed modem telecommunications, and are more particularly directed to adaptive frequency domain equalizer circuits, systems, and methods for discrete multitone based digital subscriber line modem.
The high-speed exchange of digital information between remotely located computers is now a pervasive part of modern computing in many contexts, including business, educational, and personal computer uses. It is contemplated that current and future applications of high speed data communications will continue the demand for systems and services in this field. For example, Video on demand (xe2x80x9cVODxe2x80x9d) is one area which has for some time driven the advancement of technology in the area of digital information exchanges. More recently, the rapid increase in use and popularity of the Global Internet (hereinafter, the xe2x80x9cInternetxe2x80x9d) has further motivated research and preliminary development of systems directed to advanced communication of information between remotely located computers, particularly in accomplishing higher bit rates using existing infrastructure.
One type of technology arising from the above and continuing to evolve is referred to in the art as digital subscriber line (xe2x80x9cDSLxe2x80x9d). DSL refers generically to a public network technology that delivers relatively high bandwidth over conventional telephone company copper wiring at limited distances. DSL has been further separated into several different categories of technologies, according to a particular expected data transfer rate, the type and length of medium over which data are communicated, and schemes for encoding and decoding the communicated data.
In each case, a DSL system may be considered as a pair of communicating modems, one of which is located at a customer site, such as a home or office computer, and the other of which is located at a network controller site, typically a telephone company central office. Within the telephone company system, this modem is connected to communicate with some type of network, often referred to as a backbone network, which is in communication with other communication paths by way of equipment such as routers or digital subscriber line access multiplexers (xe2x80x9cDSLAMsxe2x80x9d). Through these devices, the backbone network may further communicate with dedicated information sources and with the Internet As a result, information accessible to the backbone network, such as Internet information, may be communicated between the central office DSL modem and a customer site having its own compatible DSL modem.
Within this general system it is also anticipated that data rates between DSL modems may be far greater than current voice modem rates. Indeed, current DSL systems being tested or projected range in rates on the order of 500 Kbps to 18 Mbps or higher. According to certain conventional techniques, the data communication rates are asymmetrical. Typically, the higher rate is provided for so-called downstream communications, that is from the central office to the customer site, with upstream communication from the customer site to the central office at a rate considerably lower than the downstream rate. Most DSL technologies also do not use the whole bandwidth of the twisted pair, reserving a relatively low bandwidth channel for voice communication, so that voice and data communications may be simultaneously carried out over the same line.
The most publicized DSL technology currently under development is referred to as Asymmetric Digital Subscriber Line, or xe2x80x9cADSL,xe2x80x9d and corresponds to ANSI standard T1.413. Despite the existence of this standard, debate and competition is still present in the art, particularly as to whether devices complying with the standard provide promise for future wide scale use and whether the standard requires revision. For example, while the standard currently contemplates a modulation technology called Discrete Multitone (xe2x80x9cDMTxe2x80x9d) for the transmission of high speed data, an alternative data transmission technique referred to as carrierless amplitude/phase modulation (xe2x80x9cCAPxe2x80x9d) has also recently gained favor in the field. In any event, given the current state of the art, it is contemplated that ADSL systems will communicate data over a single copper twisted pair at downstream rates on the order of 1.5 Mbps to 9 Mbps, and with an upstream bandwidth will range from 16 kbps to 640 kbps. Along with Internet access, telephone companies are contemplating delivering remote local area network (xe2x80x9cLANxe2x80x9d) access and VOD services via ADSL.
Other DSL technologies being developed include High-Bit-Rate Digital Subscriber Line (xe2x80x9cHDSLxe2x80x9d), Single-Line Digital Subscriber Line (xe2x80x9cSDSLxe2x80x9d), and Very-high-data-rate Digital Subscriber Line (xe2x80x9cVDSLxe2x80x9d). HDSL, unlike ADSL as described above, has a symmetric data transfer rate, that is, it communicates at the same speed in both upstream and downstream directions. Current perceived speeds are on the order of 1.544 Mbps of bandwidth, but require two copper twisted pairs. However, the operating range of HDSL is more limited than that of ADSL, and is currently considered to be effective at distances of approximately 12,000 feet or less, beyond which signal repeaters are required. SDSL delivers comparable symmetric data transfer speed as HDSL, but achieves these results with a single copper twisted pair which limits the range of an SDSL system to approximately 10,000 feet. Lastly, VDSL provides asymmetric data transfer rates at much higher speeds, such as on the order of 13 Mbps to 52 Mbps downstream, and 1.5 Mbps to 2.3 Mbps upstream, but only over a maximum range of 1,000 to 4,500 feet.
Of course, in addition to performance considerations and to the distance over which DSL communications may be carried by conventional twisted-pair infrastructure, the cost of the modem hardware is also a significant factor in the selection of a communications technology. It is therefore contemplated that a lower data rate technology may provide high-speed data communications, with downstream data rates exceeding 1 Mbps, over existing twisted-pair networks and at cost that is competitive with conventional non-DSL modems, such as 56 k, V.34, and ISDN modems.
By way of further background, in one aspect of DSL modems implementing DMT modulation, it is required that a modem receiving a signal correct or equalize the received signal given variations which are imposed on the signal due to various factors such as the transmission medium as well as the extensive circuitry and processing which is imposed on the signal as it is received and is deciphered by the receiving modem. In this regard, a typical modem equalizes the signal using a time domain equalizer separate from a frequency domain equalizer. The time domain equalizer shortens the channel impulse response to reduce the inter-symbol interference. The frequency domain equalizer provides a correction function which equalizes the channel amplitude and phase distortions at each sub-carrier frequency. As detailed later in connection with the preferred embodiment, however, one drawback in a non-ideal approach may sacrifice information at one end of the frequency spectrum (e.g., lower frequency sub-carriers) in favor of information at the other end (e.g., higher frequency sub-carriers). Therefore, a need arises to address this drawback as well as the considerations discussed above.
In one embodiment, there is a method of modem communications between first and second modems over a communications facility. The method operates the first modem to issue communications to the second modem over the communications facility. These communications comprise a plurality of subchannel signals. The method also operates the second modem to perform various steps. In one of these steps, the second modem converts the communications from time domain communications to frequency domain communications, where the frequency domain communications signals comprise a plurality of subchannel signals. Each of these plurality of subchannel signals comprises an amplitude portion and a phase portion. In another of these steps, the second modem equalizes the amplitude portion of each of the plurality of subchannel signals using fixed gain factors corresponding to each of the plurality of subchannel signals. In still another of these steps, the second modem equalizes the phase portion of each of the plurality of subchannel signals using adjustable phase factors corresponding to each of the plurality of subchannel signals. The adjustable phase factors are adjusted in response to previous communications from the first modem to the second modem. Other circuits, systems, and methods are also disclosed and claimed.